The present invention relates to high quality of service transmission of Asynchronous Transfer Mode (ATM) information over noisy, high speed wireless links. More specifically, the invention relates to protecting ATM cells transmitted over high speed wireless channels using framing, interleaving and forward error correction coding schemes to correct burst errors.
Asynchronous Transfer Mode (ATM) is a method for transmitting information via a broadband Integrated Services Digital Network (B-ISDN). ATM is a transfer mode which uses fixed size data units, known as cells, made up of 53 bytes; 48 of these bytes contain data (referred to as the information field or payload) and 5 contain overhead (referred to as the cell header). The data bytes correspond to the ATM Adaptation Layer (AAL) and the overhead bytes define the ATM cell header. The header includes cell routing information and an error detection/correction mechanism to protect the routing information. Also included in the AAL is an error detection mechanism to mitigate against transmission errors in the AAL data.
In principle, ATM cells may be transported using any digital transmission format. However, the International Telecommunication Union (ITU) approved synchronous digital hierarchy (SDH) and plesiochronous digital hierarchy (PDH) signal formats are of particular interest from an international standardization viewpoint. An important feature in SDH transmission is the inclusion of pointer bytes which are used to indicate the location of the first byte in the payload of a SDH frame and also avoid slips and their associated data loss due to slight phase or frequency variations between the SDH payload and frame. ATM cell transmission via SDH further incorporates a cell delineation mechanism for acquisition and synchronization of ATM cells on the receiving side of the network. There exist, however, a number of factors which present problems when transmitting ATM cells in SDH and PDH formats over wireless links.
Transmission considerations and performance criteria for both ATM and its carriage in the SDH and PDH frames are currently based on the assumption that transmission bit errors are randomly distributed. This assumption is generally valid for most optic fiber based transmission systems. However, due to the necessity of including channel coding into wireless transmissions to reduce receiving station size, such as in satellite channels for example, and thereby minimize ground segment costs, transmission bit errors in a wireless environment are likely to occur in bursts.
Furthermore, ATM protocols assume that the transmission medium has very low Bit Error Rates (10−12 or better) and that bit errors occur randomly. Most satellite and wireless links have much worse Bit Error Rate characteristics (10−6 to 10−8) and errors tend to occur in long bursts, especially when Viterbi coding is used in the modems.
These factors have a severe effect on ATM protocols and applications that use ATM. For example, ATM cells use a feature to correct single bit errors in the cell header. This feature is useless in the satellite and wireless bursty error environments, with the net result being that the cell headers of a large number of cells can get corrupted, leading to cell losses or misinsertions.
Therefore, in wireless environments, such as satellite communications, the bursty nature of transmission errors requires a careful evaluation of transmission performances of both the SDH or PDH transport signal and the ATM cells contained therein.
U.S. Pat. No. 5,600,653 teaches a technique to solve the problem of cell losses by selectively interleaving the ATM cell headers. The interleaving and deinterleaving of cell headers distributes the possible burst of errors in the ATM cell header over a number of cell headers such that no ATM cell header would have more than one bit in error. A Header Error Correction (HEC) algorithm is then used to correct the one bit error occurrences in all ATM cell headers thereby reducing the ATM cell loss to an acceptable level. This technique, however, does not address the problem of high ATM cell error ratio (CER) characteristics present in wireless links.